The primary interest in RPN for computing revolves around its nature
as a stack machine.

Stack machines are very easy to write and involve a very small number
of data structures. The most important one of these is the stack.

When a RPN language encounters a token that is not an operator, it
simply pushes that token onto the stack. Consider the first three
statements of the following program:

2 3 1 + - top
^ +-----+
| 1 |
+-----+
| 3 |
+-----+
| 2 |
+-----+

The parser/interpreter read a '2'. The '2' is not part of the known
set of operators (+ - * /) and thus it pushed it on to the stack.
It next read a '3'. Likewise, this was not part of the known set
of operators and so it pushed it onto the stack also. Each 'push'
is done by placing the new value on the top of the stack. This
forms a LIFO or FILO structure (they mean the same thing). The same
is true for the '1'.